1. Field of the Invention
The present invention relates to a magnetoresistive effect element and a magnetic random access memory.
2. Description of the Related Art
Conventionally, various solid-state magnetic memories are proposed. In recent years, a magnetic random access memory (MRAM: Magnetic Random Access Memory) using a magnetoresistive effect element which exhibits a giant magnetoresistive (GMR) effect has been proposed. In particular, a magnetic random access memory using a ferromagnetic tunnel junction which exhibits a tunneling magnetoresistive (TMR) effect has been attracting attention.
An MTJ (Magnetic Tunnel Junction) element of a ferromagnetic tunnel junction has a three-layer film including a first ferromagnetic layer/an insulating layer/a second ferromagnetic layer. In a read state, a tunnel current flows through the insulating layer. In this case, a junction resistance changes depending on a cosine of a relative angle between magnetizations of the first and second ferromagnetic layers. Therefore, the junction resistance is a minimum value when the magnetizations of the first and second ferromagnetic layers are parallel to each other, and is a maximum value when the magnetizations are antiparallel to each other. This is called the TMR effect. The change in resistance by the TMR effect may exceed 300% at room temperature.
In a magnetic memory device including a ferromagnetic tunnel junction as a memory cell, at least one ferromagnetic layer is regarded as a reference layer (or a fixing layer or a pin layer), a magnetization direction of the ferromagnetic layer is fixed (or invariable), the other ferromagnetic layer is regarded as a recording layer (or a magnetic recording layer, a free layer, a variable layer), and the magnetizing direction thereof is made invertible (variable). In this cell, pieces of binary information “0” and “1” are associated with the parallel and antiparallel arrangements of magnetizations of the reference layer and the recording layer to store information. In a write state of the recording information, a magnetization of a storing layer is inverted by a magnetic field generated when a current is caused to flow in a write wire arranged independently of the cell. Alternatively, the device is directly energized to invert the magnetization of the storing layer by a spin torque injected from the reference layer. The reading is performed in such a manner that a current is caused to flow in the ferromagnetic tunnel junction to detect a change in resistance by the TMR effect. A large number of the memory cells described above are arranged to configure a magnetic memory. With respect to an actual configuration, switching transistors are arranged to the cells, respectively, as in, for example, a DRAM to select an arbitrary cell, and a peripheral circuit is built therein.
As a magnetic memory device using the spin torque, an in-plane magnetization type magnetic memory device having a magnetization headed in a film surface direction is known. However, in the in-plane magnetization type magnetic memory device, when an in-plane medium using a Co—Cr-based alloy as used in a hard disk medium is used, a magnetic anisotropy in the film surface direction is largely dispersed because a crystal axis rotates. For this reason, in a large-capacity memory, the characteristics of the devices widely fluctuate, which is not desirable.
On the other hand, a magnetic memory device using a magnetic recording layer having a perpendicular magnetization has been proposed (for example, see JP-A 2002-261352 (KOKAI)). When a material having a perpendicular magnetization is used, even though crystal grains rotate in a film surface, a magnetic anisotropy in a direction perpendicular to the surface does not fluctuate because the crystal axis is perpendicular to the film surface. For this reason, by using the perpendicular magnetization type magnetoresistive element, memories the characteristics of which hardly fluctuate can be advantageously realized.
However, as a problem caused when a perpendicular magnetization film is used, the presence of a multi-domain state is given. In general, the perpendicular magnetization film has a narrow magnetic domain wall because the perpendicular magnetization film has a larger magnetic anisotropy. Since the magnetization is perpendicular to the film surface, a gain of magnetostatic energy generated by forming domains is large. Because of these properties, in the magnetoresistive effect element having a perpendicular magnetization film, a multi-domain state remains as a stable state, and an intermediate-value state or the like remains as a problem.
Furthermore, in order to realize a large-capacity memory, devices must be micropatterned, and a degree of cell occupation in a chip must be increased. However, since the thermal agitation resistance of a memory cell is determined by a magnetic anisotropy and a device volume, the thermal agitation resistance is deteriorated when the devices are micropatterned. For this reason, a sufficient record holding characteristic cannot be obtained. When the multi-domain state is present as described above, the multi-domain state becomes a metastable state, and deterioration or the like of the effective thermal agitation resistance is further conspicuous.